The present invention concerns a doctor arrangement for a rotary intaglio printing machine.
In a rotary intaglio printing machine the printing ink which is to be transferred on to the material to be printed such as a web or sheets of paper is applied to the plate cylinder of the machine by a procedure wherein the plate cylinder which rotates at a high speed of rotation dips with a region of its peripheral surface which extends over the entire length of the plate cylinder into a bath of printing ink which is contained in an ink trough or fountain. When that happens the depressions in the peripheral surface of the plate cylinder are filled with printing ink. As excessive printing ink adheres to the peripheral surface of the plate cylinder at regions of the surface which do not have depressions, after the peripheral surface of the plate cylinder issues from the bath of printing ink, it is absolutely essential to arrange at the printing cylinder a device known as a doctor, by means of which that excess printing ink can be scraped off the plate cylinder before the respective region of the surface of the plate cylinder passes into the printing zone where it comes into contact with the web of material to which the printing is to be applied.
A conventional doctor is usually in the form of a relatively stiff metal blade, the length of which is somewhat greater than the axial length of the associated plate cylinder. Such a doctor is rigidly clamped in position in an equally long and massive doctor bar, by means of which it is so positioned beside the plate cylinder that it can be pressed against the peripheral surface of the plate cylinder by means of a hydraulic or pneumatic device, along a generatrix which extends along the plate cylinder parallel to the axis of rotation thereof.
In order to avoid local overheating of and thus damage to the surface of the plate cylinder as a result of the pressure applied by the doctor to the peripheral surface of the plate cylinder, the entire doctor arrangement, that is to say the doctor together with its doctor bar and the entire pressing assembly, can be periodically moved to and and fro by a few centimeters in a direction parallel to the axis of the plate cylinder.
However that doctor arrangement suffers from a number of disadvantages. For example the durability of such a doctor is limited to about 500,000 to 700,000 revolutions of the printing cylinder, even in completely trouble-free operation. That remains that, when the machine is required to perform a very long print run which can involve producing a printed product in very high numbers which can be the order of magnitude of up to for example twelve millions, the doctors of all the printing mechanisms of the machine have to be replaced a number of times during the operation of printing such a number of items. For that purpose it is necessary for the rotary intaglio printing machine to be stopped until the doctors to be changed have been removed and replaced by new doctors. The machine stoppage times caused thereby result in a delay in finishing the respective print run to be produced, and give rise to increased costs. In addition a rotary printing machine which operates at high speed cannot be abruptly stopped and quickly accelerated to full speed again from a stopped condition. While the speed of rotation is being reduced and raised again prior to and after a change of doctor, a large amount of waste occurs, as it is in practice not possible to keep the register condition of the various printing mechanisms properly adjusted with the required degree of accuracy during the periods in which the speed of rotation of the intaglio printing machine is varying.
The above-discussed problem is aggravated by the fact that at any time, that is to say even after just a few hundreds of revolutions, a doctor can suffer from damage due to a lump of ink or the like which is pressed through between the peripheral surface of the plate cylinder and the doctor. Such damage makes itself noticeable straightaway because the web of material to which printing is to be applied can be seen to have a continuous ink stripe which extends to and fro in a zig-zag configuration, because of the above-mentioned reciprocating movement of the doctor. Hitherto, in such a situation the rotary intaglio printing machine had to be immediately stopped and the damaged doctor had to be replaced, which also gives rise to the above-described disadvantages.